PRINCIPLES OF TOXICOLOGY - Biology East Borneo

536 CONTROLLING OCCUPATIONAL AND ENVIRONMENTAL HEALTH HAZARDSto be distinguished from sampling strategy, fall into two broad categories: direct-reading instrumentationand laboratory-based analytical methods.Direct-reading instruments (also known as real-time analysis) provide an immediate measurementof levels of the given air contaminant. They are widely used in confined space entry programs,emergency response situations and hazardous-waste sites, and as supporting evidence for the laboratorybasedanalytical methods. Examples of direct-reading instruments include electrochemical cells, metaloxide cells, infrared gas analyzers, portable gas chromatographs, and detector tubes (also calledlength-of-stain colorimetric tubes). While some of these instruments can be worn by employees toobtain an 8-h TWA, most are used to conduct area sampling. They are excellent for determining leaksand other sources of air contaminants. They usually need to be calibrated with a test gas of knownconcentration immediately before and after use. The chief disadvantage of this family of techniquesis that the minimum level of detection is much higher than the laboratory-based techniques. This isdue to the availability of more sensitive equipment in the laboratory and the ability to “preconcentrate”the contaminant onto some type of sampling media.Sampling techniques that require laboratory analysis are most often employed where low limits ofdetection are required and when integrated exposure information is acceptable. For example, activatedcharcoal is used to capture organic solvents, which are then analyzed by desorbing the charcoal incarbon disulfide or another appropriate solvent, taking an aliquot of the solution, and injecting it intoa gas chromatograph for quantification. Other forms of laboratory analysis include ion chromatography,atomic absorption (for metal dusts), X-ray diffraction (for silica), inductively complex plasmaemission spectrometry, mass spectrometry (particularly useful for identifying unknown air contaminants),scanning electron transmission microscopy (for asbestos), and a host of others.One final form of air sampling relies on passive diffusion, instead of active pumping of air throughdetectors or media. These devices are often marketed as “badges” or “tubes,” and can rely oncolorimetric techniques or postsampling laboratory analysis. They are relatively easy to use andinexpensive. However, they are susceptible to error in areas of excessive air turbulence. Additionally,they may fail to record short bursts or peak exposures due to the longer time interval required bydiffusion. Few have been validated independently.The final stage of evaluation involves the analysis of interviews, observation of work practices, andair or physical agent monitoring results. In its simplest form, evaluation means comparing the measuredexposure levels with OSHA PELs or the current TLVs ® . At best, this only demonstrates possiblecompliance with OSHA PELs. At worst, it can provide a false sense of security because exposuresbelow published exposure limits are not guarantees that an individual will not suffer adverse healtheffects.Thorough evaluation includes an analysis of operating procedures, work practices, and “thenumbers” to arrive at recommendations for reducing exposures to the lowest feasible level. Forexample, air monitoring of a parts cleaning operation might indicate that vapor concentrations insideworkers’ breathing zones are below the TLV ® for the solvent. To some, this may suggest that no furthermeasures are needed. To a competent industrial hygienist, however, the observations that the parts aremuch smaller than the tank and that workers always leave open the lid on the solvent tank suggestssimple changes in design and work practices that could lower exposure levels. By splitting the lid sothat the size of the lid is only as large as needed for the largest parts, keeping the other side closed atall times, and by instructing employees to close the lid when not in use, exposure levels can be reducedsubstantially at very little cost. In short, even though the level was below published exposure limits,control measures should still be specified based on direct observation of job performance.Control Several types of control measures have already been described earlier in the discussions ofanticipation, recognition, and evaluation. In fact, the choice of various control measures is usually alogical extension of the earlier efforts.Important compromises are frequently made when workable solutions are finally identified. Trulyeffective control measures are those which do not excessively interfere with the performance of thejob to the point where workers refuse to use them, and that are not so costly that they threaten the